Another Barrel Heating Study:
M.L. McPherson, March 2005
SYNOPSIS: As was demonstrated in an earlier study, contrary to folklore, barrel heating is chiefly the result of deformation of the steel that results from the rapid rise and fall of pressure within the barrel when one fires a cartridge; hence, the barrel exterior does not continue to get hotter after the shot is fired, as it would if friction and incandescent heating were primary sources of barrel heating. This study irrefutably reinforced that conclusion – in every instance, the barrel cooled continuously after we fired the last shot. What we also proved here was that the efficiency of the chambering, with regard to how much unburned propellant follows the bullet into the bore is a critical factor in overall elastic barrel deformation and heating – more efficient case designs result in less barrel heating (and, likely, these also generate less throat damage) and in less disruption of the shooter’s sight picture. This demonstration is of critical significance to varmint hunters (and some types of target shooters) because it demonstrates that performance is not directly related to degree of barrel heating!
For a better understanding of why the various patented aspects of the SMc™ cartridge design improve performance, readers may want to review related articles covering the SMc cartridge design, as previously reported in these pages. While that theory is outside the scope of this report, it significantly explains the results reported here. Meanwhile, both reduced barrel heating and reduced sight picture disturbance resulting from use of an SMc cartridge design is critical to the success and enjoyment of varmint hunters in many situations.
The question was, “Can you demonstrate that a more efficient deign, according to the SMc theory, results in less barrel heating, as your field experience seems to indicate?” The answer was, “I only know one way to find out.” Hence, I found myself accumulating barreled actions, stocks, scopes, ammunition, a temperature-sensing gun, a can of flat-black spray paint and all manner of shooting accessories.
The result of this experiment, while tentative (I do believe that I need to repeat the study as I am almost certain that I obtained less than perfect data on at least one of the tests), is conclusive; in general, cartridge design does matter and short-fat, sharp shouldered designs (efficient) are vastly superior to long-shinny designs. Specifically, the patented SMc design works as advertised. Not only does it provide increased muzzle velocity but also generates significantly less barrel heating than any conventional design of similar performance.
For the test reported here, I had the able assistance of my friend, Roger Hazlewood. Without his help, I should not have been able to do this testing with any precision. Please note that any data errors or errors in the description of the test and the results given here are mine. Hazlewood did the shooting and the data recording. I watched the clock, measured data and did the necessary gunsmithing between tests.
What we tested and how we tested it
For this experiment we used superior factory ammunition from Hornady and Black Hills and two 5mm/35 SMc handloads using the same brand of propellant (Ramshot, a ball-type of superior quality), and the same Hornady bullets used in the factory 204 Ruger loads. We also tested two 5/35 handloads using Hodgdon extruded propellants; one with H322 and the 40-grain Hornady V-Max; the other with BenchMark and moly-plated Berger 30-grain LTBs. All tested loads, excepting the one with the Berger bullet, used naked bullets. Excepting the Berger bulleted handload (which was certainly on the hot side), we prepared all handloads, as nearly as was feasible, to simulate factory-load pressures.
We used factory Savage barreled-actions chambered in 204 Ruger, 223 Remington, 22-250 Remington, and 5/35 (we have an experimental factory-chambered Savage 1/12 twist barrel). We also used a modified factory plastic stock with the foreend removed so that the stock does not protrude forward of the receiver. This was done to insure that the barrel was neither insulated by the stock nor heated by either reflected or reradiated heat from the stock.
For the same reasons, we fitted a scout scope so that the scope did not significantly protrude forward of the receiver. We also had on hand a standard Leupold VX-III scope, a standard factory plastic stock, and a laminated wood low-profile factory stock. The reason for these was that we wanted to compare our tested barrel heating results to the barrel heating in a real gun in a standard varmint hunting configuration. However, we discovered that it was essentially impossible to get good data in the confines with these stocks and this cope installed.
We hope to soon complete a study in this regard through application of a strain gauge and seven thermistors on at least two barrels and with an interface to a computer so that we can monitor barrel heating in real time as the bullet passes through the bore.
We had painted each barrel flat black because this improves accuracy of the temperature-sensing gun that we used. This gun displays temperature in 0.2-degree C increments but repeatability accuracy is closer to 0.5-degrees C. However, with the large number of samples taken, generally, such errors will tend to cancel.
Generally, I swept the sensing cone of the gun across the barrel slowly enough so that I was able to read and report highest displayed temperature but fast enough so that barrel cooling was not significant during the reading interval. As indicated in the timing table, I took readings at a five-second cadence, starting over the chamber and working toward the muzzle. After taking the 24-inch location reading, we paused until one-minute had passed since the first chamber reading and repeated the readings, starting at the chamber. For most of the tests, we continued this sequence until we had seven readings per location, which was certainly sufficient to allow us to back-calculate temperature at each measured location to the instant after the last bullet had passed.
We fired all shots in the controlled environment of the Cortez, Colorado Rifle and Pistol Club, Indoor Range. To the extent possible, ambient temperature, air currents and lighting were maintained at a constant value. Greatest noted variable was ambient temperature, which we continuously monitored. I corrected all data presented here to mitigate variables associated with both variations in ambient temperature and variations in barrel temperature before we fired the first shot of each test string. We gathered sufficient data with each test to assure that only minimal errors resulted from the necessary corrections.
For each test, we recorded ambient temperature, then barrel temperature at the specified locations (above chamber – as close to the barrel nut as was feasible, at 6-inches, 12-inches, 18-inches, and 24-inches from the receiver). We then fired twenty rounds at a strict fifteen-second cadence – each shot occurred no more than +/- one-second from this interval; variation of most intervals was far less than +/- one-half second.
As noted above, fifteen seconds after firing the last shot of each string, we began the temperature reading series. We then took readings as indicated in the following table:
After gathering this data for each test, I used ambient temperature, pre-test barrel temperature, and timed temperature reading data as a basis to generate a relative barrel-cooling trend for each barrel at each location. I then calculated this trend backward in time to the instant we had fired the last shot for that test. Data presented below represents strictly that information – difference in barrel temperature at each location immediately after firing the last shot and before firing the first shot.
I do not represent this data as being flawless but I am confident that the general trends are valid and that no significant or meaningful systematic errors occurred in the test protocol or occur in the following graphic presentations.
We began all testing with fully cleaned barrels that I had treated with a friction-proofing agent. We ran two clean and dry patches through each barrel before beginning this test but we did not clean any barrel during this testing. All barrels were brand new 26-inch Savage, varmint profile, unfluted but otherwise finished, factory-produced to standard specifications (the 5/35 is a factory barrel chambered to our proposed standard specifications for this chambering).
Propellant used in tested factory loads is reported to be produced by PB Clermont, as marketed by Ramshot, which is the propellant brand used in the ball-powder SMc handloads used in this study. Since these propellants are all chemically very similar, differing only in burn speed, mostly as a result of differences in granule size, significant differences in barrel heating cannot be attributed to differences in propellant, at least not among the loads using Ramshot propellants.
The following graphic presentation compares barrel heating that occurred in each barrel as measured at each indicated location and in terms of degrees centigrade temperature increase. Note apparent anomalies in several of these curves suggesting slower cooling toward muzzle end; we have no good explanation for this but we certainly cannot rule out experimental error. Since barrel damage from overheating is never seen toward muzzle end of a bore, we can reasonably assume that any such measurement errors that may have occurred at the 24-inch point are not all that meaningful – what matters most is barrel heating over and immediately forward of chamber!
Subsequent testing, with temperature measurements on the muzzle, proved that the muzzle of the barrel heats dramatically more than any other portion of the barrel does. This results from the lack of material forward of the muzzle, so the muzzle has bridging strength only rearward, it therefore expands and contracts much faster, in response to bullet passage and gas venting. Faster expansion and contraction results in greater heating. This almost certainly explains the slower cooling of the 24-inch point on the barrel seen in the tests reported here. Extra heat from the muzzle soaks rearward in the barrel, slowing cooling toward the muzzle.
Note that the 5/35 SMc (curves marked with squares) generally produced barrel heating that was similar to heating produced by both the 223 Remington (curve marked with diamonds) and the 204 Ruger (curves marked with triangles). This was the result, despite a massive performance advantage for the 5/35 SMc. Not surprisingly, the 22-250 Remington (curve marked with circles) generated dramatically more barrel heating than any other tested load.
Refer to the following graph: Evident increased cooling rate at chamber end results from heat loss into receiver. Remember that we had fired twenty shots, at a cadence of fifteen-seconds per shot, before this data was taken. Hence, during the interval between firing the first shot and firing the last shot, sufficient time passed for considerable heat to pass into the receiver.
The first following is a graphic presentation of data based upon a calculation comparing barrel heating to absolute performance, as measured in terms of muzzle energy. Those loads producing more muzzle energy for any given amount of barrel heating are obvious. This presentation most fairly represents the fact that increased performance has traditionally gone hand-in-hand with increased barrel heating and a commensurate reduction in the number of shots that one could fire before pausing to allow the barrel to cool. In this analysis, the ball-powder 5/35 loads (left 2 horizontally shaded bars) significantly outperform the 204 Ruger (vertically shaded bars). This is an excellent graphic presentation of the advantage offered by the design efficiency of the 5/35 – the latter offers a significant velocity edge without a significant increase in barrel heating! Formula used for this graph is: (1.93 x ME) / delta T.
The second following is similar to the first following presentation, this graph includes bullet BC – assuming use of ideal bullets in each load (bullets similar to Nosler Ballistic Tip varminting bullets). This presentation more fairly compares effective potential performance of each load and chambering, with an emphasis on trajectory. One might call this the “What the varmint hunter gets” comparison. Here, quite obviously, the 5/35 design (horizontal bars) outperforms all others by a significant margin. Since the 55-grain Nosler BT is a superior choice for the 22-250, we have included a hypothetical calculation for that bullet (rightmost bar in each set) simply so that we do not inadvertently misrepresent potential 22-250 performance.
Formula used for this graph is: (V2 x BC) / (delta T x1570.)
A casual glance at the “Average Performance” graph suggests one fact that is obvious to experienced varminters. While the 22-250 gives a usefully flatter trajectory than does the 223, the 22-250 also heats the barrel so much faster than the 223 that heating can create a significant problem when shooting conditions are good. Conversely, the striking advantage shown by the 20-caliber (5mm) numbers, compared to either of these popular 22s is likely something of a surprise to many readers.
A tiny part of this differential stems from the fact that the 5mms have a slightly thicker barrel wall (0.010″ at any given location), which results in a tiny decrease in deformational heating resulting from any given stress loading – however, this difference amounts to only a couple of percentage points. The big heating difference stems from the fact that these particular 5mms both do a comparatively better job of trapping unburned propellant in the case.
Consider the 223 versus the 204. In this set, the 204 has a usefully sharper and wider case shoulder – such a design will trap a far greater percentage of propellant that was not ignited by the primer within the case (primer flash only ignites propellant within about one-half inch of flash hole – refer to this author’s primer plume penetration study, elsewhere on this site). The predictable result is that the 204 will show less barrel heating because less total mass will accelerate into the bore. For the same reason, even when loaded with the same weight bullet, the 204 will generate far less “felt” recoil and sight picture disturbance.
Now, compare the 5/35 to the 22-250. Here the difference is significantly more dramatic because the 5/35 does a far better job of trapping propellant in the case – the 22-250 has a relatively shallow shoulder angle while the 5/35 has a relatively steep shoulder angle (the elliptical design is also a better trap than the conical design, even when average angle is similar). Also, with the 5/35, the propellant plug that shears out from the unignited propellant mass and thereafter begins to push the bullet into the bore is far shorter (it contains less mass) simply because the 5/35 case is far shorter than the 22-250 case. Finally, other patented characteristics of the SMc design contribute to early ignition and rapid combustion of the entire charge mass – this both improves performance and reduces amount of unburned propellant that is accelerated into the bore. As has been independently demonstrated in ballistics laboratories, the SMc is an idealized design; the superior performance shown here is a solid demonstration of this fact!
What This Means to Varmint Hunters
The issue of reduced barrel heating for any given performance level is obvious – any experienced varmint hunter will appreciate the value this provides. There is, however, an additional and related advantage that is even more significant to most of us because, for various reasons, we prefer to observe bullet impacts.
Therefore, let us briefly visit the issue of “felt” recoil. Here, for varminting purposes, I am chiefly considering disturbance to sight picture – how far away must the target be located for the shooter to see the impact? We will surely agree that seeing is everything and the closer one can see impacts the better.
Refer to the following graph to verify that the most rapid rearward acceleration of the gun occurs shortly after the bullet begins to accelerate into the bore. This is true, regardless of how the shooter may hold the gun; inevitably, this is also the interval when the gun is least bonded to the shooter.
As gun initially accelerates rearward, it compresses the tissues of the shooter’s shoulder as it also compresses any recoil pad material; hence, as the gun moves rearward the mass of the shooter’s shoulder is progressively more solidly tied to the mass of the gun. The further back the gun moves during this initial acceleration phase, progressively, the further back the shooter’s shoulder must move and the greater the effective mass in rearward acceleration. Explicitly, the further the gun recoils, the harder it is for the accelerating bullet and propellant
Similarly, during this process the gun progressively becomes more intimately tied to the shooter. Moreover, the tighter the gun is bonded to the shooter the more the shooter can prevent the gun from rotating. Since the center of mass of the gun is not aligned with the bore axis, the gun will inevitably rotate during recoil. The butt will move downward and the muzzle will move upward. The tighter the butt is bonded to the shooter’s shoulder, the less such downward movement will occur. Hence, it is during the initial acceleration of the gun that the shooter is least able to control any non-axial acceleration – inevitably, if the gun initially accelerates faster, the sight picture will be more disturbed. Anyone who has compared sight picture disturbance with various bullet weights in the same gun is familiar with this – lighter bullets allow one to see impacts at closer ranges, despite those bullets having less time of flight toward such targets.
Therefore, because initial gun acceleration is less when less unburned propellant accelerates into the bore (indistinguishable from firing a lighter bullet), comparatively, case designs that reduce the amount of propellant that follows (actually pushes) the bullet into the bore will generate less sight picture disturbance, even if overall performance is similar.
Recent field comparisons between otherwise identical guns chambered in 223 and 5/35 (even the scopes were the same model!) dramatically demonstrated this fact. The 223, launching a 40-grain bullet at about 3900 fps with a stiff charge of Benchmark generated more apparent recoil and sight picture disturbance than the 5/35 load that was launching a 40-grain bullet at about 4200 fps, using a few grains more of the same propellant. In repeated tests, whoever fired each gun agreed that it was possible to clearly see impacts at a closer range when firing the 5/35 and that this gun produces less felt recoil.
For this same reason, efficient case designs will seem to generate less recoil because it is the maximum acceleration rate that determines how painfully the gun hammers into the shooter’s shoulder. This explains a situation that is widely reported by shooters who have compared similarly performing loads in otherwise identical guns chambered for the 300 Win Mag versus the 300 WSM, the later is universally reported as generating notably less recoil.